1. Field of the Invention
This invention relates to a system for controlling the internal temperature of an externally heated furnace such for example as semiconductor diffusion furnace. It is also directed to thermocouple means useful for such a system.
2. Description of the Prior Art
Prior art system for controlling the temperature of a semiconductor diffusion furnace is arranged such that the temperature of a heater provided externally of the furnace is detected to obtain an electrical signal, which in turn is compared with a present reference value, thus controlling the heater temperature to a value corresponding to the present invention reference value. Such a system works effectively when it is employed in such a circumstance that a temperature difference as great as about 10.degree. to 20.degree. C is allowed between the exterior and the interior of the furnace. In actuality, however, the semiconductor diffusion furnace requires more precise temperature control since even as small a temperature variation as about 0.5.degree. to 1.degree. C which tends to result from gas flow variation, will have adverse effect on the yield; hence, the aforementioned prior art system is not satisfactory. Another disadvantage of the conventional system is that a long time is taken before such a small temperature change as mentioned above can be externally detected, since a great heat resistance exists between the furnace temperature and the heater temperature.
In an attempt to detect temperatures at various points which are remote, thermocouples are commonly used which correspond in number to those points and are long enough to extend as far as such points. Obviously, however, it is undesirable to use such thermocouples since each of them is formed of two different types of metal which are expensive in themselves.